In an attempt to eliminate volatile organic compounds (VOCs) in the pressroom, water-based alternatives are being sought for ink formulations. Water-based printing inks for use in flexographic printing processes are known in the prior art. This type of printing process utilizes printing plates wherein the printing images stand up in relief, i.e. the areas to be printed are raised above the non-printing areas. Printing by the flexographic process requires relatively low pressure while sufficient pressure is applied to transfer the ink from the face of the image carrier to the surface of the substrate. Examples of useful water-based flexographic printing inks are disclosed in U.S. Pat. No. 4,173,554 and The Printing Ink Manual, edited by R. H. Leach and R. J. Pierce, pages 571-576, 5th edition, (Blueprint, 1993).
Water-based inks for gravure printing are also well known. In the gravure process, the printing image is engraved into a cylinder in the form of cells which become filled with ink. Printing is achieved by passing the substrate between the gravure cylinder and impression roller under pressure. Examples of useful water-based gravure printing inks are disclosed in U.S. Pat. Nos. 4,954,556 and 5,098,478.
The offset lithographic printing process presents a unique challenge to ink formulators since such process utilizes a planographic printing plate, i.e. the image and non-image areas are in the same plane on the image carrier, and two fluids are concurrently utilized.
It is fairly simple to define an image area by raising it above the background as in the case of the flexographic printing plate or lowering it as in the case of the gravure printing plate; avoidance of ink adhering to the non-image area is not too difficult to achieve. However, when all areas are on the same level, techniques must be utilized to insure that ink adheres only to the image area, and not to the non-image area.
In conventional offset lithographic printing processes, the plate is damped before it is inked with an oil-based ink. Typically, the damping process utilizes a fountain solution such as those described in U.S. Pat. Nos. 3,877,372, 4,278,467 and 4,854,969. Water will form a film on the hydrophilic areas (i.e. the non-image areas) of the printing plate, but will contract into tiny droplets on the oleophilic areas (i.e. the image areas). When an inked roller containing the oil-based ink is passed over the damped plate, it will be unable to ink the areas covered by the water film (the non-image areas), but will emulsify the droplets on the water-repellant areas (the image areas) and these will ink up. Such process is called offset lithography because the inked image on the plate does not directly print onto the paper substrate, but is first “offset” onto a rubber blanket, and transferred therefrom onto the paper substrate.
As mentioned above, conventional offset lithographic printing processes entails the use of oil-based inks and water-based fountain solutions. The ink/water balance is critical and is quite demanding of the pressman's skills. This issue is one of the several disadvantages associated with such printing processes as compared to flexographic and gravure printing processes. Moreover, the oil-based inks and aqueous fountain solutions typically employed in conventional offset lithographic printing processes contain fairly high levels of undesirable volatile organic compounds (“VOCs”).
U.S. Pat. No. 3,356,030 discloses the use of a water-based printing ink in respect to a method of planographic printing utilizing a standard lithographic printing plate. However, the patented method also entails the use of a volatile hydrocarbon fountain solution which will coat the non-image areas and which is re-applied between successive printings. Of course, the use of a volatile hydrocarbon fountain solution undermines the principal purpose of the water-based ink compositions, i.e. the avoidance of the use of volatile organic compounds (“VOCs”) during the printing process. Indeed, the water-washable ink compositions of the present invention are already low VOC offset lithographic printing inks which can be printed with any fountain solution whatsoever, and in addition will reduce the VOC further in the pressroom by eliminating the use of solvents during clean up.
In the 1980s, a resurgence of interest occurred in respect to “waterless” lithographic printing processes. Both positive and negative waterless planographic printing plates are commercially available from Toray Industries of Japan. The image area of a waterless planographic plate is a photopolymer similar to that employed for the image area of a conventional plate. However, the non-image area is coated with a polymer such as a silicone which is ink repellant. Further information about waterless printing plates and processes may be found in U.S. Pat. Nos. 5,370,906 and 5,417,749.
The waterless printing process solved two issues: VOCs emanating from the fountain solutions and control of the ink/water balance by the pressman. However, the difference in surface energy between the image and non-image areas of the conventional offset lithographic printing plate is typically 40 dynes/cm is dramatically reduced to 20 dynes/cm in the case of the waterless printing plate. Therefore the latitude between scumming and poor print density is considerably narrowed and the issue of VOCs (emanating from the oil-based ink) still remains in respect to waterless printing.
German Offenlegungsschrift DE 41 19 348 A1 pertains to a moistureless offset printing method and a water-based printing ink. The ink described therein is one which will adhere to hydrophilic materials, but not to hydrophobic materials, and contains a dye, water, 5-50% water-soluble macromolecular binder and a hygroscopic liquid, preferably a multihydric alcohol.
U.S. Pat. No. 5,725,646, which is incorporated herein by reference, describes a way of stabilizing a waterbased offset ink composition without drying up on a conventional multi roller ink train. Such composition eliminated the principal disadvantages of conventional offset lithographic printing inks, viz. high levels of VOCs emanating from the oil-based ink and the aqueous fountain solution and the difficulty in controlling the ink/water balance, while preserving the principal advantage of the conventional lithographic printing process, i.e. high surface energy differential between the image and non-image areas of the printing plate. It consists of using a rewetting agent, namely hydroxy ethyl ethylene urea. Since this rewetting agent does not dry, it remains in the final film making it susceptible to poor water resistance. Use of conventional rewetting agents, such as glycols and glycol ethers not only imparts poor water resistance to the final film, but also increases the tack of the formulations, thereby limiting its use on high speed presses.
U.S. Pat. No. 6,444,021, which is incorporated herein by reference, describes water washable color newspaper inks that contain a modified soybean oil-based resin, a pigment, an acid neutralization agent, and a humectant. This combination of ingredients causes the ink to readily disperse in water or aqueous detergent solutions that are commonly used for cleaning. The modified soybean oil-based resin provides the water washability when the oil is neutralized in the ink formulation providing the washable characteristics of water. These changes significantly expand coverage possible for newspaper inks.
U.S. Pat. No. 6,444,022, which is incorporated herein by reference, describes a water based lithographic ink comprising water; a modified rosin polymer comprised of resins soluble in water regardless of the pH of the water, resin rosin salts soluble in water at pH ranging from 7.5 to 10 and aqueous emulsion resins; a modified linseed oil; and pigment. This modified linseed oil comprises the reaction product of a polyglycol and linseed oil and provides an alternative to the use of hydroxyethyl ethylene urea which use has the disadvantage of slow drying and poor water resistance.
U.S. Patent Publication US 2002/0083865 which is incorporated herein by reference, describes a water based lithographic ink composition containing water, a resin-bonded pigment, a nonionic surfactant, a rewetting agent, and a polymerizable surfactant. This combination of ingredients allows the lithographic ink to dry rapidly at a rate comparable to oil-based inks, to emit minimal or no volatile organic compounds and to be water washable.
U.S. Pat. No. 6,709,503, which is incorporated herein by reference, describes a water based heatset offset lithographic ink containing water, polyamide resins or fumarated rosin resins, hydroxyethylene urea as a humectant, a modified linseed oil, a dibutylated benzoguanamine, a pigment and p-toluene sulfonic acid. This type of inks eliminates volatile organic compounds from the press room and the need for a humidity chamber in a manner which does not rely upon modified soybean oil based resin.